Silica aerogels differs from urethane foams (PU), foamed polystyrene (EPS), and vacuum insulation panels (VIPs), in that there are almost no changes in their heat-insulation performance across the ages. Furthermore, silica aerogels have heat resistance of up to about 400° C. For these reasons, silica aerogels have attracted a great deal of attention as next-generation heat-insulation materials.
With regard to PUs and EPS that are obtained through foaming with a gas having a low heat conductivity, their heat-insulation performance deteriorates as the gas comes out of the materials over time. Moreover, PUs and EPS have poor heat resistance. VIPs have excellent heat-insulation efficiencies of several milliWatts per milliKelvin. However, over time, trace amounts of molecules of air penetrate into VIPs from their portions that have been bonded through thermal fusion bonding when core materials are vacuum-encapsulated, resulting in loss of vacuum, and therefore causing problem of degradation across the ages. Furthermore, there is also a problem that VIPs have a heat resistance of only about 100° C.
Silica aerogels are superior to any other existing heat-insulation materials in terms of deterioration with age and heat resistance. Silica aerogels have excellent heat conductivities of around 15 mW/mK. However, silica aerogels have network structures in which silica particles on the scale of several tens of nanometers are connected in rows through point contact. Accordingly, silica aerogels do not have sufficient mechanical strength. Therefore, in order to overcome this weakness, studies have been made to improve the strength by way of combining silica aerogels with fibers, unwoven fabrics, resins, etc.
In general, inorganic nanoporous materials such as silica aerogels are synthesized by the sol-gel method, which is a liquid-phase reaction. Water glass (an aqueous solution of sodium silicate) or alkoxysilane compounds such as tetramethoxysilane are used as raw materials. These materials, and a liquid medium such as water or alcohols, and, as needed, a catalyst are mixed, and are hydrolyzed. That is, the materials are subjected to polycondensation in a liquid medium to thus form a wet gel. Then, the wet gel is subjected to a silylation reaction. Finally, the liquid medium inside the wet gel is evaporated to dry the gel. Synthesis of inorganic nanoporous materials are described in WO/2007/010949, JP-A-7-257918, and JP-A-2003-183529.